Due to their high leaf area index, natural forests maintain high transpiration fluxes (thick dark blue arrow), which exceed the evaporation fluxes
over the ocean (thin dark blue arrow). The evaporated moisture undergoes condensation and disappears from the gas phase. Air in the atmospheric column above the forest rarifies. As a result, ascending air motion develops over the forest canopy, which, in turn, "sucks in" moist air from the ocean (light blue arrow). It then returns to the ocean in the upper atmosphere (dotted arrow) after precipitation of moisture
over the continent.

Boreal biotic pump in action: in winter, when the forest is dormant, precipitation is concentrated over the ocean; over
land it markedly declines from west to east.
In summer, when the forest is active and draws moisture from the ocean, rainfall predominantly occurs over land.
Now it does not decline with distance from the ocean, but remains approximately constant over seven thousand kilometers!
(Figure 5 from Theoretical and Applied Climatology 2013)

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Pump Physical Basis

Partial pressure of water vapor cannot exceed the saturated one (red line), the latter drops twofold for each
ten degrees of temperature decrease. In the static atmosphere water vapor partial pressure must drop twofold
for each 9 km of height increment (blue line). When the red line finds itself below the blue line,
as is the case at the observed lapse rate of 6.5 K/km, the atmosphere cannot be static.
Water vapor condenses and disappears from the gas phase. There appears a dynamic air flow from areas with less intense condensation
to areas where condensation is more intense.